Such a unit with a single imaging element and a transparent substrate arranged as a plane-parallel plate with a diffractive injection element and a first and second diffractive extraction element which are arranged at a distance from one another is described in T. Levola: “Diffractive optics for virtual reality displays”, Journal of the SID 14/5 (2006), pages 467 through 475. The diffractive injection element is used for binocular beam path splitting and the diffractive extraction elements are used among other things for enlarging the exit pupil of the optical system of the described display unit in comparison to the exit pupil of a collimation lens arranged between the imaging element and the plate.
The disadvantageous aspect in the described unit is that as a result of the diffractive injection element and the diffractive extraction elements different chromatic aberrations are impressed on the image for the right eye and the left eye of the user. Chromatic aberrations shall be understood here as being especially so-called field-dependent binocular color cast which occurs for the reason that the light energy impinging on the eye pupil from a pixel of the imaging element depends on the position of the pixel in the imaging element and on the wavelength, and is in addition different for the right and left eye of the user. This leads to the consequence according to the state of the art that one and the same pixel is perceived with different brightness in each eye depending on the color. The user thus perceives an image with different color distortion in each eye (“field-dependent binocular color cast”). Since the substrate with the injection element and the diffractive extraction elements are arranged as symmetrically as possible, the color defect perceivable by the user is mirrored. When the user perceives a red color cast at the right edge of the image in the right eye, he automatically perceives a red color cast at the left edge of the image in the left eye.